Over the last decade the fraction of energy added to utility grids by wind parks has increased significantly. Therefore, there is also an increased need for controlling wind parks delivering energy to utility grids with respect to a number of parameters which are important for the utility grid to work faultlessly. In addition to the voltage and the frequency of the power delivered to the utility grid, it may also be important to provide, on request of the utility system operator, a certain reactive power or a certain power factor. The reactive power is the power needed by inductive and capacitive users to build up their magnetic and electric fields, respectively. With an alternating current such magnetic and electric fields will be built up and down periodically, which leads to a reactive power flow from and to the electric generators.
The power factor is the ratio of the active power over the apparent power. The apparent power is the square root of the square of the active power plus the square of the reactive. A power factor of one means that no reactive power is present and the supplied power contains only active power and corresponds to a phase difference between the voltage and the current of zero degrees, since the power factor is given by the cosine of the phase angle between the voltage and the current. On the other hand, a phase difference of 90°, i.e. a power factor of zero, means that only reactive power is present.
Variations in the demand for reactive power in the utility grid need to be compensated through reactive power provided by the power generators. As long as the power delivered to the utility grids by wind parks was only a minor fraction of the total power delivered to the grids, there was no need for the wind parks to react to the reactive power demands or power factor demands of the grid. However, with the always increasing fraction of power fed into utility grids by wind parks, the dynamic power factor control or a dynamic reactive power control of wind parks becomes more and more important.
Dynamic power factor control of wind parks is often implemented with capacitor banks mounted in individual wind turbines. A varying number of capacitors are selectively connected to the grid to provide such reactive power as is required to meet the specified power factor. Dynamic power factor control may also be implemented by an arrangement where the individual wind turbines are equipped with a power electronic converter that converts part or whole of the electricity supplied by the wind turbine. The power electronic converter of the individual wind turbine is programmed to control the power factor of the electricity supplied by the wind turbine. E.g., if the wind turbine is equipped with a transformer for transforming the voltage output of the converter to a medium voltage on a bus connecting the wind turbines of the wind park, the reactive power output from the wind turbine can be controlled by setting a voltage set point of the converter connected to the primary side of the transformer.
For these types of power factor control system the desired power factor is typically provided as a signal from a central SCADA (Supervise Control and Data Acquisition) system of the wind park. This SCADA system is also referred to as the wind park controller. The utility system operator dispatches a request to the wind park for a certain power factor and the central SCADA system sends set points in accordance with the power factor request to the individual wind turbines to compensate for the power factor contribution of the wind park electrical infrastructure. The wind park controller is connected to the individual wind turbines via communication lines. These communication lines may have significant lengths because the wind park may be distributed over a very large geographical area.
An arrangement for dynamic power control is, e.g., described in U.S. Pat. No. 5,083,039, which describes a wind turbine with dynamic power factor control and sending control signals to power electronic converters of wind turbines. The power electronic converters are then locally controlled such that the power factor delivered by the local wind turbine is shifted through changing the ratio of active and reactive current supplied to the grid by the converter module of the power electronic converter.
In a wind park the power output of each wind turbine constantly changes due to local variations in wind speed, wind direction etc. Since the amount of reactive power provided by a single wind turbine of a wind park changes with its active power output but the total amount of reactive power requested from the wind park by the utility system operator usually remains constant over relatively longer periods of time, the reference set points of all wind turbines need to be adjusted frequently by the wind park controller.
Wind parks are often located in areas with unreliable communication infrastructure. Currently, when communication between the wind park controller and a specific wind turbine fails, a wind turbine will either preserve its last valid set point or it may fall back to a predefined set point after a predetermined time-out on the communication with the wind park controller. However, if the power output of the wind turbine changes due to e.g. variations in wind speed while its voltage set point is kept constant, so will its reactive power. Thus, a wind park may provide an amount of reactive power which largely deviates from the amount requested by the utility grid operator when there is a loss of communication within the wind park. Such unrequested amounts of reactive power may affect operation of the utility grid. Similarly, if the wind turbine is controlled in reactive power control mode and the reactive power set point is maintained during loss of communication, significant changes in the voltage level at the point of interconnection will result when power output changes. Again, this may affect utility grid operation. Furthermore, when the communication is restored, a large reactive step or reactive ramp event may occur in the general control of the wind park due to the power generation changes that have occurred while the communication was lost.
Thus, it is an object to provide an improved method of operating a wind turbine.